We have https://www.wikidata.org/wiki/Property:P6824, though this is not integrated into the interface.

Maybe possible alternate values could be displayed too (we don't have a property for that yet).

Proposed it at Wikidata:Property_proposal/replacement_value

Created as P9729.

Note that it’s also possible to have only a replacement value but no replacement property. Query of conflicts-with constraints with replacements. (Also, the message documentation should make it clear to translators that “instead” needs to be as vague as it is, because looking at that query, it seems like $3 should sometimes be used “instead of $1” and sometimes “instead of $2” – this needs to be left to the interpretation of the human reader of the message, I think.)

For the multi-multi case I would suggest a message with more than one paragraph, something like:

An entity should not have statements for both $1 and $2.

Replacement properties:

• …

Replacement values:

• …

This is probably a separate message; I’m not sure how many of the other options should be combined into shared messages (using PLURAL) and how many of them should also be separate messages.

In T277826#7926652, @Lucas_Werkmeister_WMDE wrote:

Note that it’s also possible to have only a replacement value but no replacement property.

In this case should I assume that the same property with a different value should be used instead?

One of the few examples currently where replacement value without replacement property is used is P9650, should the message in this claim be something like this?

An entity should not have a statement for Internet Game Database company ID if it also has a statement for instance of with value company. Use instance of with value video game developer instead.

If the same property should be used not repeating the "instance of" property in the replacement part makes the message more confusing, because it's not that clear if the property that needs a change is the one that triggered the constraint message or the other one.

Additionally, we could also make that if "replacement value" is provided "replacement property" would be mandatory for the constraint to be valid, that way there wouldn't be confusion about which of the properties should have its value replaced.

I would prefer to avoid interpreting the constraint too much, and limit the message to something like “Use the value video game developer instead.” The user can hopefully make a better decision about what a correct fix looks like. (But yes, making the replacement value invalid when no replacement property is given would also be an option, I think.)

In T277826#7926652, @Lucas_Werkmeister_WMDE wrote:

For the multi-multi case I would suggest a message with more than one paragraph, something like:

An entity should not have statements for both $1 and $2.

Replacement properties:

• …

Replacement values:

• …

How can I do a message with multiple paragraphs as the one in the example above? The only way I can think of is adding HTML code to the messages, but checking the list of the existing messages in the extension I don't see any message with HTML tags in it, so I suppose there's a better way to get the same result.

Include two line breaks in the source (\n\n – you can often see that in qqq.json before the list of parameters for a message, for example), then it’ll be wikitext-parsed as a paragraph break.